FACT: 400 million was changed from US currency to other currencies and then transferred to Iran. Obama admitted to the transfer, the only question is why it was done!
It was done because we owed them the money. Iran paid 400M for fighter jets. The money was held in trust, and the arms deal was signed, but then the revolution happened. The planes were never delivered and the U.S. has been sitting on the money ever since. The U.S. owed Iran the 400M plus interest. Iran had sought a much larger restitution in international court.
Iran didn't demand ransom for release of hostages. They've been fighting in international court to get their money back for decades. The U.S. did use the money it owed as leverage settling negotiations with Iran.